What is Si unit: Definition and 26 Discussions

The International System of Units (SI, abbreviated from the French Système international (d'unités)) is the modern form of the metric system. It is the only system of measurement with an official status in nearly every country in the world. It comprises a coherent system of units of measurement starting with seven base units, which are the second (the unit of time with the symbol s), metre (length, m), kilogram (mass, kg), ampere (electric current, A), kelvin (thermodynamic temperature, K), mole (amount of substance, mol), and candela (luminous intensity, cd). The system allows for an unlimited number of additional units, called derived units, which can always be represented as products of powers of the base units. Twenty-two derived units have been provided with special names and symbols. The seven base units and the 22 derived units with special names and symbols may be used in combination to express other derived units, which are adopted to facilitate measurement of diverse quantities. The SI also provides twenty prefixes to the unit names and unit symbols that may be used when specifying power-of-ten (i.e. decimal) multiples and sub-multiples of SI units. The SI is intended to be an evolving system; units and prefixes are created and unit definitions are modified through international agreement as the technology of measurement progresses and the precision of measurements improves.
Since 2019, the magnitudes of all SI units have been defined by declaring exact numerical values for seven defining constants when expressed in terms of their SI units. These defining constants are the speed of light in vacuum, c, the hyperfine transition frequency of caesium ΔνCs, the Planck constant h, the elementary charge e, the Boltzmann constant k, the Avogadro constant NA, and the luminous efficacy Kcd. The nature of the defining constants ranges from fundamental constants of nature such as c to the purely technical constant Kcd. Prior to 2019, h, e, k, and NA were not defined a priori but were rather very precisely measured quantities. In 2019, their values were fixed by definition to their best estimates at the time, ensuring continuity with previous definitions of the base units. One consequence of the redefinition of the SI is that the distinction between the base units and derived units is in principle not needed, since any unit can be constructed directly from the seven defining constants.The current way of defining the SI is a result of a decades-long move towards increasingly abstract and idealised formulation in which the realisations of the units are separated conceptually from the definitions. A consequence is that as science and technologies develop, new and superior realisations may be introduced without the need to redefine the unit. One problem with artefacts is that they can be lost, damaged, or changed; another is that they introduce uncertainties that cannot be reduced by advancements in science and technology. The last artefact used by the SI was the International Prototype of the Kilogram, a cylinder of platinum-iridium.
The original motivation for the development of the SI was the diversity of units that had sprung up within the centimetre–gram–second (CGS) systems (specifically the inconsistency between the systems of electrostatic units and electromagnetic units) and the lack of coordination between the various disciplines that used them. The General Conference on Weights and Measures (French: Conférence générale des poids et mesures – CGPM), which was established by the Metre Convention of 1875, brought together many international organisations to establish the definitions and standards of a new system and to standardise the rules for writing and presenting measurements. The system was published in 1960 as a result of an initiative that began in 1948, so it is based on the metre–kilogram–second system of units (MKS) rather than any variant of the CGS.

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  1. ehchandlerjr

    A Question about cgs vs SI units in the context of the Debye Length

    Hello - I am trying to understand the physical meaning the undergirds the Debye length as it pertains to different unit systems. I understand that fundamentally its the distance at which the distribution of ions doesn't differ by more than the effect of k_B*T from the rest of the solution...
  2. C

    How Can I Refresh My Knowledge of Physics?

    Hi, I love physics since school, but after years I forgot a lot of what I studied. I hope this forum will help refresh my memory. Also I built a web converter application to convert different measurement units and this forum can be really helpful for me to promote it and keep improving it. Thanks
  3. GoodQuestion

    B Vacuum permittivty with alternative Standard-Unit for meters

    I was wondering: what would be the value of Vacuum Permettivity in the case 1 meter (say 1m") would be defined as the distance we nowadays see as 1,10 meters. At first this looks easy: ##\varepsilon0 = 8.8541878128 \cdot 10{^{12}}## F / m with normal meters so ##\varepsilon0" = \varepsilon0...
  4. bagasme

    Why Isn't Horsepower an SI Unit, Despite Its Popularity?

    When we talk about engines on automotive industry (cars, motorbikes, trucks, etc.), we often refer the power produced by such engines by horsepower. But when we talk about electricity, the power is measured in kilowatts. Despite the former case is highly popular, why isn't horsepower considered...
  5. L

    SI Units: Is the combination of SI units still an SI unit?

    Homework Statement Is N/m^2 a si unit I know Newton is and meters but togeather are there or is m^2 Homework Equations The Attempt at a Solution N/m^2 m^2
  6. F

    I Why am I getting two different results in emu and SI unit?

    I am computing force between two magnetic poles each of one unit pole (in emu) and situated one centimeter apart. In electromagnetic units: ##F_{dyne}=\dfrac{p^2}{r_{cm}^2}=\dfrac{1^2}{1^2}=1 dyne## where ##p## is pole strength in emu In SI units: ##F_{N}=k_A \dfrac{P^2}{r_m^2}=10^{-7}...
  7. Nim

    I Why Is There No SI Unit For Information?

    Why is the quantity "information" missing from the SI system? Also, if they did add it, do you think it would be added as a base unit or dimensionless derived unit?
  8. S

    I How to Convert Cgs to SI Units for Flux and Energy Values?

    Hi I have a value that is 10.7 mJy, that I need to convert to SI units. I thought it would be 1.7*10^(-2)*10^(-26) but that might be wrong? Then I have a flux value, kappa, of 2 cm^2.g^(-1) that needs to be converted to meters and kilogram. I thought it was 2*10^(-7) m2/kg, but that might be...
  9. Timizorzom

    Paper Plane Distance Experiment: Power & Energy Calculation

    Hi, I'm doing an experiment to get my silver CREST award, in which I've created a mechanical paper plane thrower. I have thrown various types of plane, but all with the same design. The thrower uses a 9V battery, but is more like 6 now. The thrower has two motors, each which take 70mA. The...
  10. A

    Why Is the Conversion Between CGS and SI Units in Electromagnetism Confusing?

    Dear friends, As you know, in CGS units we have vacuum permeability as unity (1). so H=B and we use Oersted for H. on the other hand, in SI, permeability is (4 x pi x 0.0000001) and we use Ampere per meter for H. unfortunately after trying for couple hours, I couldn't derive the algebra of...
  11. at94official

    Converting 1,602 ×10^−19 C to SI Unit Prefixes

    Just simple questions and I need clarifications why is that your answer please. 1,602 ×10^−19 C write to SI Unit Prefixes
  12. CKaiL

    Photoelectric Equation - SI unit question

    Homework Statement I am trying to plot a graph of stopping voltage against frequency The given equation is eV0 = hf - w0 So i need to change it to the form of y = mx + C in order to plot this graph V0 = h*f/e - w0/e V0 = h/e * f - w0/e -- > V0 = y , m(gradient) = h/e , x= f and -w0/e = C (y...
  13. N

    Solving SI Unit Analysis: Force & Area

    Homework Statement When an object falls through air, there is a drag force that depends on the product of the surface area, A (m2), and the square of the velocity, in (m/s). The equation is Fdrag = CAv2. The metric unit of force is the Newton, or (N). 1 N = 1 kg*m/s2. What are the units...
  14. N

    SI Units for Pressure: N m^2 or N m^-2?

    Are the SI units for pressure N m^2 or is it N m^-2?
  15. M

    Unit Conversion: SI to British Units with Real-Life Consequences

    Homework Statement This is the question Still think that unit conversion isn't important? Here is a widely publicized, true story about how failing to convert units resulted in a huge loss. In 1998, the Mars Climate Orbiter probe crashed into the surface of Mars, instead of entering orbit...
  16. R

    Why is there no SI unit for sound?

    The deciBel is probably the most commonly used unit for sound, but why isn't there an SI unit? Also, what is a logarithmic unit? Thanks.
  17. Y

    Some help with Si Unit conversions

    Some help with Si Unit conversions :) Homework Statement 762 Mg/m to ng/µm Homework Equations Si unit conversions The Attempt at a Solution How do I know where to place the variables? Can someone give me a tut or written explantion on how to do this?
  18. B

    The SI Unit Henry: Magnetic Induction & Voltage

    The SI unit "henry" Is a "henry" only produced by magnetic induction, in which case would produced one volt that allows the induction of one ampere per second?
  19. T

    Why Ampere is Not a SI Unit? A Brief Overview

    Why ampere is not a S.I unit?
  20. M

    The Absurdity of 1 Mol as an SI Unit - What's the Real Explanation?

    1 Mol = SI Unit - What the...!? This is ridiculous. How can 1 Mol be considered an SI-BASE-UNIT although it is simply a factor 1 Mol = 6.022 * 10²³ Every attempt to explain this thorough misconception is naturally predestined to fail! I'm not looking for an explanation, neither going to...
  21. F

    Why is the SI unit for acceleration m/(s^2)?

    Why is the SI unit for acceleration \frac{m}{s^2}(meters per second squared) when it is actually \frac{m}{\frac{s}{s}} (meters per second per second). Isn't the part concerning the seconds different? Wouldn't this give you different answers sometimes, or does that usually never get in the way.
  22. W

    SI Unit Conversion Help: Mass Density of Aluminum and Alcohol

    Homework Statement Ok, this isn't hard, I'm just thrown off by the question. From the book: "The quantity called mass density is mass per unit volume of a substance. Express the following in SI units:" a. Aluminum, 2.7*10^-3 kg/cm^3 b. Alcohol, .81g/cm^3 Homework Equations...
  23. K

    What is the SI unit for refractive index?

    I am doing my homework and i come across a question. What is the SI unit for refractive index?All my revision books did not have the answer.Or perhaps the book is wrong by any chance?
  24. B

    SI unit for luminous intensity

    This is the SI unit for luminous intensity. The definition relates to blackbody radiation emitted at a certain temperature for a certain material (so I guess it ISN'T blackbody!), platinum, I think. Except... I don't understand the necessity for the introduction of this unit. Isn't...
  25. W

    What is the Definition of the SI Unit of Force?

    Hello, What is the definition for Newton
  26. Adrian Baker

    Is the milliHelen the new unit for measuring beauty?

    Apparently it has been decided that the new unit for beauty will be the milliHelen. One milliHelen is said to be the beauty required to launch one ship. Tee Hee